1. Technical Field
The present invention relates generally to nanoparticles for optoelectronic applications, and, more particularly, multilayered plexcitonic nanoparticles and layer-by-layer assembly methods of producing same for controlling the resulting plasmon-exciton coupling strength of the synthesized nanoparticles.
2. Description of the Related Art
Silver (Ag) and gold (Au) nanoparticles have been studied extensively for their unique optical properties in the ultraviolet, visible, and infrared regions of the electromagnetic spectrum. These properties arise from the collective oscillation of conduction band electrons throughout the particle in response to optical excitation, a process commonly termed as localized surface plasmon resonance. Resonance occurs when the frequency of incident radiation is at or near the frequency of the electronic oscillation. This resonance results in a strong enhancement of the local electric field. This property may be used in applications such as surface-enhanced Raman scattering (SERS), surface-enhanced fluorescence, and the study of nonlinear optical response. Additionally, the size and morphology of the silver and gold nanoparticles have a significant impact on surface plasmon resonance frequency and therefore have a great significance in such fields as sensors and photonic devices.
Cyanine dyes are commonly used in spectral sensitization and have potential application in novel optoelectronic materials. Structurally, these dyes consist of two heterocyclic units that are connected by an odd number of methine groups. The color of the dye is primarily determined by the length of the polymethine chain. A particularly intriguing property of cymine dyes is their tendency to aggregate under certain conditions in solution. These so called J-aggregates exhibit a narrow absorption band that is red-shifted with respect to the monomer absorption band. The shift in absorption of the aggregate has been described by a Frenkel exciton model in which excited states are formed by the coherent coupling of molecular transition dipoles.
Multilayered nanoparticles, composed of both a noble metal and a J-aggregate dye, provide a unique framework for studying plasmon-exciton interactions. Numerous structures exhibiting these plasmon-exciton interactions have been fabricated and studied in recent years. Some nanostructures have involved the direct adsorption of J-aggregate dyes onto the surface of sliver nanoparticles with varying geometries. One example of this is the work reported by J. Hranisavljevic, N. M. Dimitrijevic, G. A. Wurtz, and G. P. Wiederrecht, “Photoinduced charge separation reactions of j-aggregates coated on silver nanoparticles,” J. Am. Chem. Soc. 124(17), 4516-4537 (2002). Other efforts have focused on the aggregation of cyanine dyes onto complex geometries. For example, cyanine dyes were adsorbed onto silica core/gold shell nanoparticles in the work reported by N. T. Fofang, T. H. Park, O. Neumann, N. A. Mirin, P. Nordlander, and N. J. Halas, “Plexcitonic nanoparticles: Plasmon-exciton coupling in nanoshell-j-aggregate complexes.” Nano Lett. 8(10), 3481-3487 (2008). In these studies, it was observed that the absorption spectra of these composite nanoparticles were not the simple sum of the absorption of the metal nanoparticle and the J-band of the aggregate. Rather, strong plasmon-exciton interactions (plexcitons) were observed, resulting in a shift in the absorption bands of the individual plasmon and exciton resonances. A more recent study by Yoshida et al. explored the impact of inserting a spacer layer or inner shell between a gold core particle (Au) and an outer J-aggregate dye layer or outer shell. This work is reported by A. Yoshida, Y. Yonezawa, and N. Kometani, “Tuning of the spectroscopic properties of composite nanoparticles by the insertion of a spacer layer: effect of exciton-plasmon coupling,” Langmuir 25(12), 6683-6689 (2009). The spacer layer used in the study was a cationic thiol which promoted the J-aggregation of anionic cyanine dyes onto the surface of the nanocomposite. The spectral line shape of these composites was dependent on the strength of the plasmon-exciton coupling between the metal core and the J-aggregate shell.
The metallic core composition and size, excitonic shell composition, degree of J-aggregate formation within the excitonic shell, and distance between the plasmonic core and excitonic shell are among the factors that play the most significant roles in controlling optical response. Metallic core particles and excitonic dyes of varying composition are readily available. Selection of core particles and excitonic dyes for a given application may be based on the intrinsic optical properties of the material. In terms of particle shape, a range of metallic core shapes such as spheres, rods, and platelets may be fabricated using well documented solution-based techniques such as that reported in P. C. Lee and D. Meisel, “Adsorption and surface-enhanced Raman dyes on silver and gold sols,” J. Phys. Chem. 86(17), 3391-3395 (1982). However, spacer layer thickness and its impact on plasmon-exciton coupling in these nanoparticle systems have not been studied systematically. The spacer layer must serve the two-fold purpose of promoting the formation of a J-aggregate shell while also creating a distance, of specific and desired dimension, between the metallic core and the excitonic shell.
Accordingly, there is a need for systematic methods to build multilayered plexcitonic nanoparticles while controlling plasmon-exciton distances. The term “plexcitonic nanoparticles” as used herein refers to the ability to control in the synthesis the plasmon-exciton coupling strength of the synthesized nanoparticles. Methods to mathematically model the behavior of such nanoparticles are also needed in order to analyze and design such materials for various applications. Such applications may include use of the multilayered plexcitonic nanoparticles in optical filters, displays, obscurants, solar cells, and other applications.